The invention concerns an antilocking system for a road vehicle with a hydraulic multi-circuit brake installation which includes at least one static brake circuit connected to an outlet pressure space of a brake unit provided for brake pressure supply to the wheel brakes of a vehicle. The outlet pressure of the brake unit can be connected to the wheel brakes of the static brake circuit by means of an electrically actuated brake pressure control valve of a brake pressure setting device for controlling brake pressure reduction, brake pressure retention and brake pressure restoration phases of the antilocking control system. At least when the antilocking control system demands high magnitude pressure reductions, the brake pressure setting device operates on the pump-back principle according to which, in a brake pressure reduction phase, brake fluid is drained from the wheel brakes subject to the control system and is pumped back into the output pressure space of the brake unit. A pumping device is provided for this purpose and includes a stepped cylinder with two bore sections of difference diameters, offset relative to one another by means of a radial housing step. A stepped piston with piston flanges of correspondingly different diameters is guided in these bore sections so that it can be displaced in a pressure-tight manner. The larger step of the stepped piston forms an axially movable boundary of a drive pressure space, which space can be connected by means of an electrically actuated antilocking system control valve arrangement to the high pressure outlet of a hydraulic auxiliary pressure source and can, as an alternative to this, be relieved to the sump tank of the auxiliary pressure source. A smaller step of this stepped piston forms the movable boundary of a functional chamber acting as a pump chamber, in pump-back operations of the brake pressure setting device. This functional chamber can be connected, via an outlet non-return valve to the brake unit pressure outlet associated with the static brake circuit and, via a inlet valve, to the wheel brakes of the static brake circuit. A return spring is provided for biasing the stepped piston into its end position associated with maximum volume of the functional chamber, which can be connected to the wheel brakes. Actuation signals necessary for the correct switching of the brake pressure control valve and the electric solenoid valve arrangement controlling the stepped cylinder drive are created by an electronic control unit which generates these signals by a process of comparing and differentiating, with respect to time, electrical output signals from wheel rotational speed sensors whose output signals are a measure of the wheel peripheral speeds of the vehicle wheels in level and/or frequency.
Broadly such an antilocking system can be considered as known from DE-OS No. 33 47 618, although the control system on the static brake circuit, the front axle brake circuit, is designed as a single wheel control system with the brake pressure control valves individually associated with each of the vehicle wheels and a common brake pressure control system on both wheels of a brake circuit is only explicitly revealed for the rear axle brake circuit, designed as an open brake circuit.
For the static brake circuit, the known antilocking system provides a low pressure reservoir which initially accepts the brake fluid drained from a wheel brake, subject to the control system, in a pressure reduction phase. This brake fluid is then pumped back by means of a hydraulically driven return pump and into an output pressure space, associated with the static brake circuit, of the brake unit. This return pump is designed in such a way that between 0.2 and 0.4 cm.sup.3 of brake fluid can be pumped back per piston stroke into the output pressure space of the brake unit. This corresponds to between 1/20 and 1/10 of that quantity of brake fluid which is forced into the brake circuit in a vehicle of the higher power class (vehicle weight approximately 1.5 tons and maximum speed approximately 200 km/h to 220 km/h) when braking occurs with the maximum pressure permitted by the brake installation design.
The valves provided as the brake pressure control valves are 3/3-way solenoid valves whose basic position is the brake pressure build-up position. These valves can be driven by a control signal of defined control current strength into a shut off position (the brake pressure retention position) and can be driven into a brake pressure reduction position by a control signal of defined higher strength (e.g. double, control current strength), in which position the wheel brakes of the static brake circuit, subjected to the control system are connected to a low pressure reservoir from which the brake fluid is pumped back into the outlet pressure space of the brake unit by means of the hydraulically driven feed pump.
In such a design of the brake antilocking system, pumping back to meet the requirements requires a drive for the hydraulic return pump with a stroke repetition frequency of at least 10 Hz which, because of the associated pressure shocks, leads to unpleasant noise in control operation. Such noise is not only disadvantageous because of the associated adverse effect on vehicle driving comfort but, above all, because particularly careful drivers who, due to a "defensive" driving style only cause the antilocking system to respond extremely rarely and are therefore not "accustomed" to such a noise, can be startled by it. This may cause them to react even worse in a moment of danger with which a braking situation requiring an antilocking system response is always associated. Also disadvantageous in this known antilocking system, is the necessary technical complication involved in achieving it. This is because of the low pressure reservoir and the complicated construction of the brake pressure control valves provided in addition to the hydraulically driven return pump.
The disadvantage of the unpleasant noise is avoided in an antilocking system known from DE-OS No. 29 08 482, in which the wheel brakes, subjected to an antilocking control, are each associated with a pressure modulator, which provides a brake pressure control action on the principle of producing brake pressure change by changing the volume of a modulator outlet pressure space to which the wheel brake cylinder of the particular wheel brake is directly connected. This modulator outlet pressure space has a movable axial boundary formed by a first piston flange of a control piston. The control piston also has a second piston flange solidly connected to the first flange by means of an elongated piston rod. This second piston flange is guided so that it can be displaced in a pressure-tight manner in a larger step of a stepped bore of the modulator housing where it forms the axially movable boundary of a reaction space. This reaction space is permanently connected to the high outlet pressure of an auxiliary pressure source and a drive pressure space, whose axially movable boundary is formed by the second larger flange of the control piston and whose axially fixed boundary is formed by an end wall of the modulator housing.It is possible to alternatively connect this drive pressure space (by means of electrically triggered inlet and outlet valves) to the high pressure outlet of the auxiliary pressure source and to its non-pressurized reservoir. A return spring is supported on the end wall of the modulator housing forming the boundary of the drive pressure space and fixed relative to the housing of the drive pressure space. This return spring acts on the larger piston flange of the control piston and biases into its basic position associated with the minimum volume of the outlet pressure space of the modulator. This minimum volume is associated with normal brake operation, i.e. brake operation not subjected to the control system. In this basic position of the control piston, a central valve, located in its smaller piston flange, is held in its open position against the action a valve spring, because its valve body is supported by a stop pin fixed relative to the housing. In this open position of the central valve, the modulator outlet pressure space (connected to the wheel brake cylinder) is connected to an inlet space which is connected to the outlet pressure space of a brake unit in which an actuating force proportional to the brake pressure can be built up by actuation of a brake pedal. The second axial boundary of this inlet space is formed by the smaller diameter step of an annular piston designed as a stepped piston, which is sealed so that it can be displaced, both against the smaller bore step of the housing bore and against the piston rod connecting together the two piston flanges of the control piston. Between the two bore steps, closed by the end walls of the modulator housing, is interposed a third bore step, which has the largest diameter. The piston is sealed against the third bore step so that it can be displaced. A radial annular piston flange, which follows on from the pistons smallest step, forms the boundary of the inlet space. At an axial distance from this flange, the annular piston has a second radial flange which is sealed, so that it can be displaced against that bore step in which the larger flange of the control piston is also guided. Thus, it too can be displaced in a pressure-tight manner. The annular piston is pan-shaped on an end facing towards the reaction space in such a way that its outer shell surrounds, at a radial distance, the piston-rod shaped central section of the control piston and, together with the latter, forms the boundary of an inner annular space. This space communicates through a transverse duct of the annular piston with an outer annular space, whose axial boundaries are formed by the two radial flanges of the annular piston. The outer annular space is permanently connected to the non-pressurized reservoir of the auxiliary pressure source. The largest diameter flange of the annular piston and a housing step form the axial boundaries of an annular space used as the drive pressure space. A valve which, due to an axial displacement of the control piston acting to increase the volume of the modulator outlet pressure space, opens into the annular space in a functional position so that it communicates with the reaction space and is therefore also subject to the high outlet pressure of the auxiliary pressure source. The valve body of this valve is designed as an essentially cylindrical sleeve which is sealed on the reaction space side against the piston rod of the control piston so that it can be displaced. Otherwise however, the sleeve outer shell surrounds the central piston rod of the control piston with a small radial distance between them so that there is an inner annular gap within the valve body communicating with the non-pressurized annular space. The valve body is sealed on the outside against the outer shell of the pan-shaped annular piston part so that is can be displaced. The sleeve-shaped valve body has, on its section sealed against the piston rod of the control piston, an external conically shaped sealing surface which forms a boundary of an external groove of the valve body. The external groove extends between this conical sealing surface and the section of the valve body sealed against the annular piston and together with the valve body, forms an annular space which is connected to communicate via transverse and longitudinal ducts with the non-pressurized annular space of the annular piston and with the annular space which can be used as the drive pressure space. The valve body is forced by a valve spring into sealing contact of its conical surface with the edge of the reaction space end opening of the annular piston.
In order to achieve a pressure reduction phase of the antilocking control system and a drive pressure reduction phase of the antilocking control system, the drive pressure space of the modulator is shut off against its reaction space and is instead connected via the outlet valve to the non-pressurized sump reservoir of the auxiliary pressure source. By this means, the control piston experiences an introductory displacement, such as to increase the modulator outlet pressure space, which is connected to the wheel brake cylinder of the wheel brake which can be subjected to the control system. In consequence, the central valve of the piston flange forming the boundary of the modulator outlet pressure space closes; the communicating connection between the non-pressurized annular space of the annular piston and the annular space and annular gap, bounded by the valve body and the annular piston is also interrupted by contact between a central stop and sealing flange of the control piston and an inner edge of the sleeve-shaped valve body. Due to the displacement of the control piston, and therefore also of the sleeve-shaped valve body, the conical sealing surface of the latter rises from the seat formed by the annular piston and the pressure present in the reaction space is also connected into the annular drive pressure space. This results in the annular piston always following the motion of the control piston, which is itself displaced so as to increase the volume of the outlet pressure space in the reaction space (if the pressure in the drive pressure space bounded by the larger flange of the control piston is relieved). On the basis of the construction of the pressure modulator and its function explained up to this point, it follows that the stroke of the control piston must be dimensioned so as to be sufficiently large for complete pressure reduction at its maximum stroke and that pump-back operation by means of the pressure modulator is impossible. Although the antilocking system known from DE-OS No. 29 08 482 has the advantage that practically no noise occurs when it responds, it does have the severe disadvantage that in no case where the antilocking system responds, is the driver given an obvious pedal reaction to indicate this fact,
A compromise, advantageous relative to the above, between undesired noise and a pedal reaction which is desirable in the case of an "extreme" response of the antilocking system (which will be correspondingly rare) is provided by Applicants' own German patent application P No. 36 37 781.3. This describes an antilocking system with pressure modulators which have a piston which can be displaced in a housing, the piston forming the boundary of an outlet pressure space which can be alternatively connected to the high pressure outlet and the non-pressurized reservoir of the modulator. This piston displacement occurs by action of the pressure present at auxiliary pressure source to provide control of the brake pressure reduction and brake pressure build-up phases of an antilocking control cycle. These pressure modulators have at least one return spring which urges the piston into a position associated with maximum volume of the primary chamber for controlling the pressure build-up and pressure reduction phases of an antilocking system control valve arrangement. The piston of the particular pressure modulator is designed as a stepped piston, whose smaller piston step forms a boundary of the outlet pressure space and whose larger piston step forms a boundary of the drive pressure space. In normal brake operation, the drive pressure space is subject to the high outlet pressure of the auxiliary pressure source so that the piston is forced against the action of a powerful return spring into its basic position associated with minimum volume of the modulator outlet pressure space. In this basic position, an inlet valve is mechanically driven into its open position in which the outlet pressure space, associated with the controllable brake circuit of the brake unit of the brake installation, is connected to communicate with the outlet pressure space of the pressure modulator.
This inlet valve is designed as a non-return valve which has already taken up its shut off position in the introductory phase of a pressure reduction stroke of the modulator piston, which is controlled by relieving the pressure in its drive pressure space. After the inlet valve has taken up this shut off position, an increasing pressure reduction occurs in the modulator outlet pressure space with further displacement of the modulator piston. The increase in the modulator outlet pressure space, attainable by means of a piston displacement, is smaller than the volume of the liquid quantity which can be forced, at maximum braking pressure, into the wheel brake(s) of the brake circuit connected to the pressure modulator. The wheel brake(s) controllable by such a pressure modulator can be shut off from the outlet pressure space of the pressure modulator by means of a solenoid valve whose basic position is the open position. If the brake pressure reduction attainable by a single pressure reduction stroke of the modulator is not sufficient for the antilocking control, the particular wheel brake is shut off from the modulator outlet pressure space and then the drive pressure space of the modulator is again subject to drive pressure so that it now operates as a return pump, which pumps brake fluid previously drained from a wheel brake back into the brake unit, in order to reduce brake pressure at the wheel brakes.
In order that braking can still take place in the case of a failure of the auxiliary pressure source, the modulator piston takes up its end position associated with the maximum volume of the modulator outlet pressure space in which the inlet valve is shut off. The modulator is provided with a mechanical controlled bypass valve which is open in the end position of the modulator piston with the faulty function mentioned. This permits connection of brake pressure from the brake unit via the modulator outlet pressure space into the wheel brake(s). However, as soon as the modulator position is displaced, even if only slightly, from its end position so as to produce brake pressure restoration or to produce pump-back operation, the bypass valve closes again. The valve body of the bypass valve is forced against its valve seat with a closing force which becomes greater as the modulator piston becomes "closer" to its position corresponding to the minimum volume of the modulator outlet pressure space. A disadvantage of this arrangement is that situation in which the auxiliary pressure source has only "partially" failed, in such a way that its outlet pressure is just insufficient to hold the modulator piston in the position corresponding to minimum volume of the outlet pressure space against the effect of the return spring and the outlet pressure of the brake unit connected into its outlet pressure space. This holds the modulation piston in a position slightly displaced from the ideal, and although the inlet valve is closed, the bypass valve cannot open and the brake installation is practically inoperative. An antilocking system with pressure modulators of the type described in the German patent application P No. 36 37 781.2 is therefore questionable for safety reasons.
The object of the invention is therefore to improve an antilocking system of the general type mentioned at the beginning in such a way that, without adverse effect on the control functions, the technical complexity necessary to achieve the system is reduced, the noise due to the control system is substantially avoided and, in addition, high functional reliability of the brake installation overall is ensured.
The invention achieves the object by having the inlet valve designed as a 2/2-way valve, controlled by stroke motions of the stepped piston 27, and held in its open position when the position of the stepped piston is associated with minimum volume of the functional chamber which can be connected to the wheel brakes. This 2/2-way valve, starting from this open position, can only reach its shut off position by a displacement of the stepped piston taking place in the sense of increasing the functional chamber. When the stepped piston has reached a position in the immediate vicinity of its end position associated with maximum volume of the functional chamber and starting from this end position, the 2/2-way valve 48 only returns to the open position by means of a displacement of the stepped piston taking place in the sense of reducing the volume of the functional chamber, after the stepped piston has executed a minimum stroke h, relative to this end position. A position indicator is provided which generates electrical output signals which are characteristic of the positions of the piston and continually vary with changes to them. The output signals of the position indicator are fed as additional information inputs to an electronic control unit which, by processing them, generates signals for switching over the antilocking control valve arrangement by which the pump-back operation of the stepped cylinder 24 can be controlled. The difference in the volumes of the functional chamber, between its maximum and minimum values, is between 1/5 and 1/3 of that volume of brake fluid which must be forced into the wheel brakes of the static brake circuit in order to achieve the maximum brake pressure permitted by the design of the brake installation.
The antilocking system of the invention is suitable for control both on the so-called select-low principle, according to which brake pressure changes caused by the control system in each wheel brake of the static brake circuit take place in the same sense and with the control system coming into action as soon as a locking tendency occurs on only one of the wheel brakes of the static brake circuit and being maintained as long as wheel brake of this static brake circuit exhibits a locking tendency; and control on the so-called select-high principle, according to which control is only initiated when all the wheel brakes of the static brake circuit show a locking tendency but in which control is maintained as long as there is a locking tendency even if on only one of the wheel brakes of the static brake circuit. A control system employing the first-name principle is advantageous where the static brake circuit is the rear axle brake circuit of the vehicle whereas control according to the second principle is suitable for a static brake circuit including both front wheels. For both types of control, the construction per brake circuit of the antilocking system uses: only one brake 2/2-way solenoid pressure control valve; a stepped cylinder with a mechanically controlled function valve, which provides automatic switching as required from pressure modulation operation to pump-back operation of the stepped cylinder;, and only one antilocking system control valve arrangement, by means of which the drive control of the stepped cylinder takes place. This construction is very simple, because it is substantially easier to control a two-position solenoid valve than a three-position solenoid valve in contrast to the known antilocking system. This alone provides a substantial control technique simplification relative to the known antilocking system. Since, from the statistical point of view, an overwhelming majority (more than 95%) of braking situations involve actuating the brake installation with a force which corresponds to at most one fourth, or a smaller fraction, of that actuation force that must be employed in order to generate the maximum brake pressure permitted by the brake installation design, the increase in functional chamber volume achievable by a single "suction" stroke of the piston of the stepped cylinder also suffices even if it is necessary in a corresponding high, proportion of brake situations requiring control to achieve the largest possible brake pressure reduction at the wheel brake(s) subject to the control system so that switching the stepped cylinder to pump-back operation only occurs on rare occasions. Therefore, in the overwhelming majority of braking actions subject to an antilocking control system, the stepped cylinder also fulfills the function of a pressure reservoir provided in the known antilocking system, which can therefore be omitted in the antilocking system of the invention. This permits a further substantial reduction in the technical complication necessary for the development of the antilocking system of the invention. The antilocking system according to the invention therefore operates in the overwhelming majority of braking situations requiring control, corresponding to the statistical frequency mentioned, by so-called pressure modulation operation, i.e. in that mode of operation in which brake fluid which is accepted in the functional chamber of the stepped cylinder (considered in this case as the modulation chamber) in order to reduce the pressure in the wheel brakes, is forced back into the wheel brakes in brake pressure restoration phases of the antilocking control system so that a single pressure reduction stroke and restoration stroke of the stepped piston is required to achieve the brake pressure changes necessary for control. This is of benefit both with respect to the sensitivity of the control system and with respect to the "control comfort" because it substantially avoids otherwise disturbing noise.
The design of the stepped cylinder, wherein a pump-back stroke of the stepped piston can pump back a quantity of brake fluid into the outlet pressure space of the brake unit whose volume is between 1/4 and 1/2 of the difference between the minimum and maximum values of the volume of the functional chamber of the stepped cylinder, allows the cylinder to act mainly in pressure modulation operation, i.e. as pressure modulator. This achieves the result that, even with suitably small spatial dimensions of the piston cylinder, a repetition of the pump-back stroke cycle of the piston of the stepped cylinder only becomes necessary on very rare occasions, which again is of benefit to control comfort.
It is advantageous if the inlet valve is designed as a spool valve whose housing, is an axial extension of the stepped cylinder, from its section forming the boundary of the functional chamber. A valve bore containing the valve body of the spool valve is designed as a bind hole starting at the bore step forming the fixed boundary, of the functional chamber The blind hole is offset by a housing step relative to the cylinder bore step, in the immediate vicinity of which the valve spool is sealed so that it can be displaced relative to the functional chamber. The valve spool of the inlet valve is provided with a radial flange at an end section located within the functional chamber of the stepped cylinder. The radial flange is, in turn, encompassed by a stop flange, pointing radially inwards of the stepped piston. These two flanges determine the maximum magnitude h of a possible relative stroke motion between the stepped piston and the valve spool. The valve spool is also provided with an end flange which forms the movable boundary of a balance space which is in continuous communicating connection with the functional chamber of the stepped cylinder used for brake pressure modulation via a longitudinal and a transverse duct of the valve spool. This end flange of the valve spool forms the boundary of the balance space at one end and also forms the valve body of the inlet valve, which in its shut off position, closes both the valve inlet duct entering the blind hole and an overflow duct leading from the blind hole to the functional chamber. This provides for a simple design means of the function switch-over valve of the stepped cylinder of the antilocking system.
The configuration of the antilocking system control valve arrangement includes two valves which can be actuated electrically and are each designed as 2/2-way valves. The basic position of one valve causes the output pressure of the auxiliary pressure source to be connected to the control pressure space of the stepped cylinder and its actuated position causes the control pressure space to be shut off relative to the auxiliary pressure source. The basic position of the second valve causes the control pressure space to be shut off relative to the non-pressurized sump tank of the auxiliary pressure source and its actuated position connects the control pressure space to the sump tank of the auxiliary pressure source. This has the advantage that when the brake pressure control valve is shut off, brake pressure retention phases of the antilocking control system can be controlled in a simple manner by means of the antilocking system control valve arrangement alone.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.